The present invention relates to a thermal printhead having heat generating resistor bodies consisting of thin film of a Cr-Si alloy subjected to a stabilization aging heat treatment.
The thermal printhead can be used, for example, in facsimiles. The thermal printer such as facsimile employs a thermal printhead constituted by a number of small heat generating resistor bodies arrayed in a row. The heat generating resistor bodies are connected to the conductor lines individually, which are adapted to supply signal currents to the heat generating resistor bodies in accordance with recording signals, so that the each heat generating resistor body generates heat selectively to effect a recording on a heat-sensitive recording paper which is held in contact with the thermal printhead.
The heat generating resistor body used as the constituent of thermal printhead can be broadly classified in accordance with the production method into two types, one is so-called thick film type while the other is so-called thin film type.
The heat generating resistor body of thick film type is produced by depositing a heat generating resistor body on an alumina substrate by a screen printing technique. The heat generating resistor body of thin film type is formed by depositing the heat generating resistor body by vacuum evaporation or sputtering on a ceramic substrate and then effecting a photo-etching.
The present invention relates to the thermal printhead of thin film type. Generally, the thermal printhead is required to meet various requirements. Namely, it is essential that the heat generating resistor bodies are stable to exhibit small fluctuation in the resistance value. In addition, since the heat generating resistor bodies are repeatedly heated to high temperatures during the recording, the heat generating resistor body should exhibit a high resistance against oxidation and small change in resistance value. Furthermore, it is important that the heat generating resistor body can operate at a high response speed in response to the recording signal. In other words, the temperature of the heat generating resistor should rise and fall sharply in response to the rise and fall of the recording pulse signal.
The thermal printhead for use in the thermal printer has a construction which includes a substrate made of a material having both of high electric insulating ability and smoothness of surface, such as glazed alumina ceramic substrate, a multiplicity of heat generating resistor bodies in the form of thin films formed on the substrate, and electric conductors for supplying the heat generating resistor bodies with the electric power. In operation, pulse current is supplied through the electric conductors to the heat generating resistor bodies corresponding to the recording signals to obtain the required heat pulse in accordance with the recording signal. The heat generating resistor bodies supplied with the electric current are activated to generate the heat to make a recording on the heat-sensitive recording paper which is held in contact with the thermal printhead.
Hitherto, the heat generating resistor bodies of the thermal printhead are made from tantalum nitride, tantalum silicon or the like materials to have a form of thin film. This heat generating resistor body of thin film type, however, is drastically oxidized at a high temperature in excess of 200.degree. C. to increase the resistance value, resulting in a degraded thickness of the printing. In order to obviate this problem, it has been proposed to coat the resistor body with an anti-oxidation protecting layer of silicon oxide (SiO.sub.2) or the like and to cover this layer with a wear resistant layer such as of tantalum oxide (Ta.sub.2 O.sub.5). This improved heat generating resistor body, however, cannot provide a sufficiently high stability in resistance value when it is driven for a long period of time.
In the meantime, there is an increasing demand for a high-speed facsimile. To cope with this demand, it is necessary that the thermal printhead operates with driving pulses of shorter pulse widths. This means that the instantaneous driving electric power is increased as compared with conventional devices, and the thin film type heat generating resistor body must produce required heat in a shorter period of time, resulting in an elevated operation temperature. Unfortunately, however, the conventional thermal printhead cannot be used practically for a satisfactorily long period of time under such a severe condition of use.